The present Invention relates generally to medical apparatus and methods. More specifically, the invention relates to apparatus and methods for optimizing cardiac resynchronization intervention, arrhythmia management, ischemia ejection, coronary artery disease management, and heart failure management.
Cardiac resynchronization therapy (CRT) is an important new medical intervention for patients suffering from congestive heart failure. When congestive heart failure occurs, symptoms develop due to the heart's inability to function sufficiently well as a mechanical pump to supply the body's physiological needs. Congestive heart failure is characterized by gradual decline in cardiac function punctuated by severe exacerbations leading eventually to death. It is estimated that over five million patients in the United States suffer from this malady.
The aim of resynchronization pacing is to induce the interventricular septum and the left ventricular free wall to contract at approximately the same time.
Resynchronization therapy seeks to provide a contraction time sequence which will most effectively produce maximal cardiac output with minimal total energy expenditure by the heart. The optimal timing is calculated by reference to hemodynamic parameters such as dP/dt, the first time-derivative of the pressure waveform in the left ventricle. The dP/dt parameter is a well-documented proxy for left ventricular contractility.
In current practice, external ultrasound measurements are used to calculate dP/dt. Such external ultrasound is used to observe wall motion directly. Most commonly, the ultrasound operator uses the ultrasound system in a tissue Doppler mode, a feature known as tissue Doppler imaging or TDI, to evaluate the time course of displacement of the septum relative to the left ventricle free wall. The current view of clinicians is that ultrasonographic evaluation using TDI or a similar approach may become an important part of qualifying patients for CRT therapy.
As currently delivered, CRT therapy is effective in about half to two-thirds of patients implanted with a resynchronization device. In approximately one third of these patients, this therapy provides a two-class improvement in patient symptoms as measured by the New York Heart Association fair level scale. In about one third of these patients, a one-class improvement in cardiovascular symptoms is accomplished. In the remaining third of patients, there is no improvement or, in a small minority, a deterioration in cardiac performance. This group of patients is referred to as non-responders. It is possible that the one-class New York Heart Association responders are actually marginal or partial responders to the therapy, given the dramatic results seen in a minority.
The synchronization therapy, in order to be optimal, targets the cardiac wall segment point of maximal delay, and advances the timing to synchronize contraction with an earlier contracting region of the heart, typically the septum. However, the current placement technique for CRT devices is usually empiric. A physician will cannulate a vein that appears to be in the region described by the literature as most effective. The device is then positioned, stimulation is carried out, and the lack of extra cardiac stimulation, such as diaphragmatic pacing, is confirmed. With the currently available techniques, rarely is there time or means for optimizing cardiac performance.
When attempted today, CRT optimization must be performed by laborious manual method of an ultrasonographer evaluating cardiac wall motion at different lead positions and different interventricular delay (IVD) settings. The IVD is the ability of pacemakers to be set up with different timing on the pacing pulse that goes to the right ventricle versus the left ventricle. In addition, all pacemakers have the ability to vary the atrio-ventricular delay, which is the delay between stimulation of the atria and the ventricle or ventricles themselves. These settings can be important in addition to the location of the left ventricular stimulating electrode is itself in resynchronizing the patient.
Some research efforts to assess cardiac motion through internal accelerometry signals have been made. For example, researchers have described the use of epicardial accelerometry for detecting arrhythmia. Kroll et al. teach a positional accelerometer for rate control (U.S. Pat. No. 6,625,493, issued Sep. 23, 2003). Mouchawar et al. disclose cardiac wall motion detection using an accelerometer for detection of arrhythmias (U.S. Pat. No. 6,002,963, issued Dec. 14, 1999). Park et al. teach the use of an accelerometer for rate adaptive pacing (U.S. Pat. No. 5,991,661, issued Nov. 23, 1999). Nilsson describes an in-can accelerometer to provide rate control (U.S. Pat. No. 6,044,299, issued Mar. 28, 2000).
Other research groups have explored the use of accelerometry in cardiac applications. Carlson et al. teach the use of an in-can accelerometer which derives pulse pressure for pacing (CRT) optimization using signals from an accelerometer and an ECG (U.S. Pat. No. 6,366,811, issued Apr. 2, 2002). Salo et al. teach the use of an accelerometer with signal processing circuitry to measure total acoustic notes to optimized CRT (U.S. Pat. No. 6,058,329, issued May 2, 2000). Cunningham teaches use of an accelerometer in the ventricle on a lead to monitor cardiac contractility (U.S. Pat. No. 6,077,136, issued Jun. 20, 2000).
Furthermore, current accelerometry in cardiac applications include implantable accelerometers for determining patient activity levels. With these devices, the pacemaker paced rate can adjust itself to allow for exercise and greater physical activity on the part of a pacemaker-dependent patient.
Recently, Overall et al. have described the concept of using apical accelerometry and other sensors to detect heart ischemia by detecting abnormalities in motion (WO 2004/066825 A2, published Aug. 12, 2004). Yu et al. have described the use of one-axis accelerometers alone to note difference in the synchronicity of ventricular wall location contractions (US 2003/0105496 A1, published Jun. 5, 2003).
Some researchers have reported the use of position sensors deployed along different aspects of the heart. Such sensors may be used to describe the extent of myocardial contraction and to effectively duplicate in part the function of ultrasonography. The parameters reported by such sensors include ejection fraction, stroke volume, cardiac output, and synchronization index. These systems typically adopt a fixed frame of reference using ultrasonographic, magnetic, or RF fields in orthogonal planes to generate a signal which can localize a catheter or catheters within the heart.
A challenge in seismocardiography is to reduce the form factor of the motion sensor which is typically implanted in a patient's heart. Conventional motion sensors and accelerometers typically have relatively large sizes. Consequently, the procedure involved in the insertion of such sensors into a patient's heart can be complex and invasive. Often, insertion of such devices, especially on a permanent basis, is impractical.
An apparatus for providing tissue movement assessment, e.g., in the form of cardiac wall timing, with a small form factor and a less invasive method of implanting such an apparatus, e.g., into a patient's heart, would be an important advancement in the art. A system for monitoring the mechanical performance of the heart in real time would have important clinical applications, such as in setting the functions of cardiac resynchronization therapy pacemakers, pharmacologic management of heart failure patients, arrhythmia detection, and ischemia detection.